1. Field of the Invention
The present invention relates to a zoom lens barrel based on the vari-focal optical system, and particularly to the improvement of the drive mechanism of the focusing optical system of the zoom lens barrel.
2. Description of the Prior Art
A zoom lens barrel based on the vari-focal optical system is intended to cease the exclusive use of the zooming lens set and focusing lens set equipped in the lens barrel, but use the zooming lens set also for focusing so that the zoom ratio is extended and focusing for a very short object distance is made possible.
However, a zoom lens barrel of this type has a different amount of displacement of the focusing optical system in the zooming operation to match with the variation of focal length between the telephoto and wide-angle positions in focusing the same object of photography.
On this account, the turn angle of the distance ring in the manual focusing operation varies unfavorably depending on the focal length.
With the intention of overcoming the above-mentioned problem, there have been proposed focusing techniques in which the operational range of the focusing cam contribution to the displacement of the focusing optical system is varied in response to the variation of focal length caused by the zooming operation, as described in Japanese patent publications JP-A-57-37307, JP-A-63-49715 and JP-A-4-184406 for example.
Any of these conventional techniques, however, cannot be rid of the occurrence of interference between the zooming and focusing mechanisms, i.e., the focusing mechanism is moved by a zooming operation. Although there has been developed a device for preventing the interference between the zooming and focusing mechanisms by using an ultrasonic motor for the auto-focusing driver so that the drive mechanism is locked after the manual focusing operation, the mechanism becomes too complex and cannot dissolve the problem completely.
With the intention of preventing the interference between the zooming mechanism and focusing mechanism in a mechanical manner, another Japanese patent publication JP-A-3-101707 discloses a zoom lens barrel of one-hand zooming type in which the focusing operation takes place by the combination of the zoom cam and focus cam.
This prior art zoom lens barrel is designed to confine the movement of the lens set, which contributes to zooming and focusing, at the intersection of the zoom cam and focus cam. Zooming is based on the relative rotation of the cams and focusing is based on the axial movement of the cams so that the relative movement of the cams at focusing is maintained virtually constant with respect to a specific object irrespective of the focal length. The zoom cam is formed on the fixed tube, while the focus cam is formed as a cam ring which couples with the fixed tube rotatably and also movably in the axial direction.
The zoom ring provided at the outermost circumference of the lens barrel is moved in the axial direction for zooming, and the distance ring provided at the same section is turned for focusing. The operating angle of the distance ring is kept constant over the range from the wide-angle position to the telephoto position at focusing.
However, the foregoing conventional lens barrels, in which the zoom cam is formed on the fixed tube, have the following problems.
The fixed tube is generally formed as a unitary member together with a bayonet for the attachment of the lens barrel to the camera body, with cam rings and moving lens holders for holding the lens sets being mounted on its interior and exterior. On this account, this component part is bulky and complex, and thus costly for manufacturing.
In the case of a fixed tube formed by metal cutting, a sophisticated machining technique is required to form a precise zoom cam, and poor yield of products is expected. In the case of an injected fixed tube, its metal mold inclusive of the zoom cam section will be very complex and expensive.